1. Field of the Invention
The present invention relates to a surface emitting laser manufacturing method, a surface emitting laser array manufacturing method, a surface emitting laser, a surface emitting laser array, and an optical apparatus including a surface emitting laser array.
2. Description of the Related Art
A vertical cavity surface emitting laser (hereinafter referred to as “VCSEL”), which is one of the surface emitting lasers, enables light to be extracted in a direction perpendicular to the substrate surface, and thus, a two-dimensional array can easily be formed merely by changing a mask pattern for element formation. Parallel processing using a plurality of beams emitted from this two-dimensional array enables an increase in density and speed, and thus, application of such two-dimensional array in various industries, e.g., optical communication, has been expected. For example, use of a surface emitting laser array as an exposure light source for an electrophotographic printer enables the printing density and speed to be increased by means of a plurality of beams.
Such electrophotographic printing requires formation of stable and minute laser spots on a photosensitive drum, and thus, stable operation in a single transverse mode and/or a single longitudinal mode is required as a laser characteristic.
In recent years, for surface emitting lasers, the method of forming a current confining structure using a selective oxidization technique to inject current only to a necessary region has been developed.
In this method, for higher performance, an AlGaAs layer with a high Al proportion, for example, an Al proportion of 98%, is provided in a multilayer reflecting mirror and selectively oxidized in a high-temperature water vapor atmosphere to form a current confining structure so that current is injected only to a necessary region.
FIG. 2 illustrates a cross-sectional diagram illustrating a surface emitting laser according to a related art example in which a current confining structure is formed using the above-described selective oxidation technique.
In a surface emitting laser 200 in FIG. 2, a vertical cavity structure is formed on a substrate 2104.
In the vertical cavity structure, an active layer 2110 that emits light upon current being injected thereto, and a lower spacer layer 2108 and an upper spacer layer 2112, which are provided to adjust the cavity length, are interposed between a lower mirror 2106 and an upper mirror 2114.
Here, each of the mirrors is configured as a multilayer reflecting mirror. In the vertical cavity structure, at least a portion of the upper mirror 2114 is etched to form a mesa structure.
In a high Al proportion layer 2115 in the upper mirror 2114, a current confining structure 2116 is formed by selective oxidation performed from the side wall of the mesa structure.
Also, the surfaces of the mesa structure are partially electrically passivated by an insulating film 2124.
A predetermined potential difference is provided between a lower electrode 2102 that is in contact with the rear surface of the substrate 2104 and an upper electrode 2126 that is in contact with an upper portion of the mesa structure. Consequently, the active layer 2110 emits light upon current being injected thereto, the light is subjected to laser oscillation by the cavity, and then, laser light is emitted from a light exit region 2140 in an aperture of the upper electrode 2126.
However, this selective oxidation is undesirable from the viewpoint of single transverse mode.
In other words, an excessively-large refractive index difference occurs because of the existence of the oxidized layer, resulting in generation of high-order transverse mode components.
Therefore, as a countermeasure for that, the method of, e.g., providing single transverse mode lasing by reducing the light emitting region to around a diameter of 3 μm by means of the current confining structure to prevent the high-order transverse mode components from being confined has been employed.
However, in such method of constraining the light emitting region, the output per element is substantially lowered because of the size reduction of the light emitting region.
In view of such circumstances as described above, conventionally, the method of enabling single transverse mode lasing while a light emitting region with a certain largeness being kept has been studied, rather than the above-described method of providing a single transverse mode by means of the current confining structure alone by narrowing the light emitting region.
In other words, the method of intentionally introducing the loss difference between the fundamental transverse mode and a high-order transverse mode to enable single transverse mode lasing while a light emitting region with a certain largeness being kept have been studied.
As one example of such method, Japanese Patent Application Laid-Open No. 2001-284722, H. J. Unold et al. (Electronics Letters, Vol. 35, No. 16 (1999)), and J. A. Vukusic et al. (IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 37, No. 1, (2001) 108) discloses a method called “surface relief”.
In this method, a stepped structure for controlling the reflectance is provided to a light exit surface of a surface emitting laser element (the light exit region 2140 in FIG. 2) to increase the loss of high-order transverse mode components over the loss of fundamental transverse mode components.
In this specification, hereinafter, a stepped structure provided in a light exit region of a light exit surface of a reflecting mirror for reflectance control as described above is referred to “surface relief structure”.
Next, a surface relief structure according to the aforementioned prior art examples will be described using FIGS. 3A to 3D. For a VCSEL mirror, in general, a multilayer reflecting mirror including a plurality of pairs of two layers having different refractive indexes alternately stacked, each layer having an optical thickness that is one fourth of the lasing wavelength (hereinafter, it may be abbreviated to “¼ wavelength” unless otherwise specified), is used. Normally, this multilayer reflecting mirror is terminated by a high refractive index layer, and thus has a high reflectance of no less than 99% using reflection by the low-refractive-index terminal interface with the air as well.
Here, first, a protruded surface relief structure illustrated in FIGS. 3A and 3B will be described. Unold et al. also discloses such a protruded surface relief structure.
Like a low reflection region 2204, which is illustrated in FIG. 3A, when the final layer of high refractive index layers 2206 (which has an optical thickness of the ¼ wavelength) is removed, the multilayer reflecting mirror is terminated by a low refractive index layer 2208. Consequently, a protruded surface relief structure can be obtained.
With such protruded surface relief structure, the phase of light reflected by the interface between the low refractive index layer 2208 and the air, which has a refractive index lower than that of the low refractive index layer 2208, is shifted by π from the phase of light totally reflected by the multilayer reflecting mirror, which exists below the interface.
Accordingly, the reflectance of the low reflection region 2204 is lowered down to, e.g., no more than 99%, and the reflectance loss can be increased up to around five to ten-fold.
In order to provide a loss difference between the fundamental transverse mode and a high-order transverse mode using this principle, as illustrated in FIG. 3B, the low reflection region 2204 is formed only around the light emitting portion so that the low reflection region 2204 and a high-order transverse mode optical distribution 2212 largely overlap with each other.
Meanwhile, a high reflection region 2202 is left in the center part of the light emitting portion so that a fundamental transverse mode optical distribution 2210 and the high reflection region 2202 in which the final layer of the high refractive index layers 2206 is left largely overlap with each other.
Consequently, the reflection loss of high-order transverse mode components is increased to suppress high-order transverse mode lasing, enabling provision of the fundamental transverse mode lasing only.
Also, like the low reflection region 2204 illustrated in FIGS. 3C and 3D, a recessed surface relief structure can be provided by further adding a low refractive index layer (which may also be a high refractive index layer) with an optical thickness of the ¼ wavelength over the final layer of the high refractive index layers 2206 (or performing etching only for the high reflection region 2202 with the low reflection region 2204 remaining as it is).
A recessed surface relief structure such as described above is also disclosed in Japanese Patent Application Laid-Open No. 2001-284722. This structure also enables lowering the reflectance by the same principle as that of the protruded surface relief, enabling provision of single mode lasing in the fundamental transverse mode only.
When control is performed by providing a loss difference between optical modes of a VCSEL by means of a surface relief structure, the alignment in the horizontal direction of the surface relief structure and the current confining structure is important. In other words, when trying to provide lasing in the fundamental transverse mode only, if the current confining aperture center and the surface relief center are misaligned from each other, extra loss will be caused in the components of the lasing mode intended to be provided (for example, the fundamental transverse mode).
As a method for manufacturing a surface relief structure, for example, the method of processing the surface by means of, e.g., FIB (Focused Ion Beam) after completion of the element is disclosed in L. M. A Plouzennec et al. (Proceedings of SPIE Vol. 3946 (2000) 219).
However, it is difficult to align the relief position and the light emitting region (current injection region) at that stage.
In particular, in the case of manufacturing a VCSEL array device, the aforementioned process, in which the amount of the aforementioned misalignment may vary depending on the element, causes a decrease in the array device yield rate. As a relief alignment technique that solves that problem, Unold et al. discloses a method called a self-alignment process.
Hereinafter, the aforementioned self-alignment process in Unold et al. will further be described using FIGS. 4A to 4E.
As illustrated in FIG. 4A, a first resist 2410 is applied onto an upper mirror 2114 of a VCSEL wafer, and patterning for a surface relief structure and patterning for a mesa structure are simultaneously performed on the resist.
Here, a protruded pattern is illustrated as a surface relief.
Next, as illustrated in FIG. 4B, the semiconductor is dry-etched using the patterned resist 2410 as a mask.
As a result of this etching, a surface relief structure 2150 is formed.
Next, as illustrated in FIG. 4C, a second resist 2420 is applied and patterned so as to protect the surface relief structure 2150. Next, as illustrated in FIG. 4D, wet etching is performed so as to form a mesa structure, making a high Al proportion layer 2115 be exposed at the side wall of the mesa.
Next, as illustrated in FIG. 4E, the high Al proportion layer 2115 is selectively oxidized to form a current confining structure 2116.
Although Unold et al. does not disclose whether or not to remove the resists 2410 and 2420 before performing the selective oxidation, here, illustration is provided supposing that they are removed. Subsequently, according to Unold et al., formation of contacts in the device, passivation, and pad bonding are performed by means of standard processes.
In this process, the positioning patterning of the mesa structure and the surface relief structure is performed using one and the same mask.
Since the current confining structure reflects the mesa structure, this process enables highly-accurate horizontal alignment of the current confining structure and the surface relief structure.